1. Networked Embedded Systems Sachin Katti
EE107 Fall 2017
Lecture 3
Timers and Interrupts
Networked Embedded Systems Sachin Katti

2. What time is the Apple Watch tracking?
How often | Granularity
Clock (all the time | sec) Alarm (all the time | sec) Stopwatch (when open | msec) Sync (all the time | sec) UI (when open | msec) Buzzer (when buzzing | msec) WiFi (when communicating | usec)

3. Why do we need timers? In this project, we need timers for
Sample analog signals using ADC
Analog signals from the microphone
1kHz means regular ADC ops with an interval of 1ms
How to measure 1ms?
Option 1: use the internal timer inside of an ADC to measure timing
Option 2: use an external timer to measure timing and interrupt the ADC module

4. Why do we need timers? In this project, we need timers for
Decode downlink message
Cloud sends control messages to the camera
The presence and obsence of WiFi packets will be detected by an envelope detector
Packet len1 encodes data one and len0 encodes data zero
Measure the length (timing) of WiFi packets to decode downlink control messages from the cloud
Use the timer capture mode to measure

5. Why do we need timers? In this project, we need timers for
Periodically adjust the reference voltage
Reference voltage fed into the radio board is important for detecting and decoding downlink control messages
We want to change the ref voltage to optimize the downlink message detection rate
Need to periodically reconfigure the reference voltage

6. Why do we need timers? In general, why do we need timers?
What time is it now?
How much time has elapsed since I last checked?
Let me know when this much time passes.
When did this external input occur?

7. What peripherals do we use to track time?
(all the time | sec) - [Alarm, Sync] 32-bit Real time clock (RTC) peripheral with interrupts (when open/buzzing | msec) - [Stopwatch, UI, Buzzer] Processor’s timer peripheral with interrupts (when communicating | usec) - [WiFi] WiFi chip’s internal timer peripheral with interrupts

8. What peripherals do we use to track time?
(all the time | sec) - [Alarm, Sync] 32-bit Real time clock (RTC) peripheral with interrupts (when open/buzzing | msec) - [Stopwatch, UI, Buzzer] Processor’s timer peripheral with interrupts (when communicating | usec) - [WiFi] WiFi chip’s internal timer peripheral with interrupts
The term is used to avoid confusion with ordinary hardware clocks which are only signals that govern digital electronics, and do not count time in human units.

9. The internal structure of a timer peripheral
Application Software
Timer Abstractions and Virtualization
Low-Level Timer Device Drivers
Software
Hardware
R/W
R/W
R/W
Compare
Counter
Capture
The purpose of the prescaler is to allow the timer to be clocked at the rate a user desires.
Prescaler
Clock Driver
Internal
External
Xtal/Osc
Frequency depends on the attached oscillator device
Figure adapted from Prabal Dutta’s EE373 slides

10. How does the number in the counter register correspond to wall clock time?
Frequency (Hz) = Cycles / Second
1 / Frequency (Hz) = Seconds / Cycle
The counter is incremented
once per cycle.
You read 100 from the counter register which is clocked by a 1 MHz oscillator. How much time has passed since the counter was reset?

11. How should we choose the OSC frequency?
For timers, there will often be a tradeoff between resolution (high resolution requires a high clock rate) and range (high clock rates cause the timer to overflow more quickly).
1MHz OSC: resolution = 1 / 1e6 second = 1us
10MHz OSC: resolution = 1/10e6 second = 0.1us
16-bits timer:
1MHz OSC: max range = 1 / 1e6 * 2^16 = ms
10MHz OSC: max range = 1/10e6 * 2^16 = ms

12. How does a firmware developer use the compare register?
Stop the timer
Set the compare register with the time it should fire
Reset the counter
Start the timer
Wait for the counter to reach the compare (via interrupt or check status reg)

13. How does a firmware developer use the capture register?
Stop the timer
Setup the timer to capture when a particular event occurs (e.g., change of GPIO pin)
Reset the counter
Start the timer
Wait for the counter to reach a capture event (via interrupt or check status reg)

14. The internal structure of a Real Time Clock (RTC)
Note: RTCs have their
own oscillator.
Why is it 32,768 kHz?
Figure adapted from Prabal Dutta’s EE373 slides

15. The internal structure of a Real Time Clock (RTC)
Note: RTCs have their
own oscillator.
Why is it 32,768 kHz?
The reason the 32,768 Hz resonator has become so common is due to a compromise between the large physical size of low frequency crystals and the large current drain of high frequency crystals.
Figure adapted from Prabal Dutta’s EE373 slides

16. How peripherals notify the CPU that their state just changed.
Interrupts
How peripherals notify the CPU that
their state just changed.
Example: UART just received a byte

17. Interrupts Definition
An event external to the currently executing process that causes a change in the normal flow of instruction execution; usually generated by hardware devices external to the CPU.
Key point is that interrupts are asynchronous w.r.t. current process
Typically indicate that some device needs service
Slides from Angela Demke Brown CSC 469H1F

18. Why interrupts? People like connecting devices
Keyboard, mouse, screen, disk drives, scanner, printer, sound card, camera, etc.
These devices occasionally need CPU service
But we can’t predict when
We want to keep the CPU busy between events
Need a way for CPU to find out devices need attention
Slides from Angela Demke Brown CSC 469H1F

19. Possible Solution: Polling
CPU periodically checks each device to see if it needs service
“Polling is like picking up your phone every few seconds to see if you have a call. …”
Slides from Angela Demke Brown CSC 469H1F

20. Possible Solution: Polling
CPU periodically checks each device to see if it needs service
“Polling is like picking up your phone every few seconds to see if you have a call. …”
Cons: takes CPU time even when no requests pending
Pros: can be efficient if events arrive rapidly
Slides from Angela Demke Brown CSC 469H1F

21. Alternative: Interrupts
Give each device a wire (interrupt line) that it can use to signal the processor

22. Alternative: Interrupts
Give each device a wire (interrupt line) that it can use to signal the processor
When interrupt signaled, processor executes a routine called an interrupt handler to deal with the interrupt
No overhead when no requests pending

23. What is the benefit of having a separate controller for interrupts?
How do interrupts work?
Interrupt
 Clear interrupt
Peripheral 1
 Peripheral P sends int X  ACK P’s int X
 Execute P’s X handler
 ACK P’s int X
Peripheral 2
Interrupt controller
CPU
Peripheral 3
What is the benefit of having
a separate controller for interrupts?
Peripheral 4

24. The Interrupt controller
Handles simultaneous interrupts
Receives interrupts while the CPU handles interrupts
Maintains interrupt flags
CPU can poll interrupt flags instead of jumping to a interrupt handler
Multiplexes many wires to few wires
CPU doesn’t need a interrupt wire to each peripheral
Fun fact: Interrupt controllers used to be
separate chips!
Intel 8259A IRQ chip Image by Nixdorf - Own work

25. How to use interrupts
Tell the peripheral which interrupts you want it to output.
Tell the interrupt controller what your priority is for this interrupt.
Tell the processor where the interrupt handler is for that interrupt.
When the interrupt handler fires, do your business then clear the int.

26. CPU execution of interrupt handlers
Wait for instruction to end
Push the program counter to the stack
Push all active registers to the stack
Jump to the interrupt handler in the interrupt vector
Pop the program counter off of the stack